| 1. |
- Ekevid, Torbjörn, 1973-, et al.
(författare)
-
Virtual Articulated Haulers : prediction of vehicle performance and design loads
- 2015
-
Ingår i: Multibody Dynamics 2015. - Barcelona : International Center for Numerical Methods in Engineering (CIMNE). - 9788494424403 ; , s. 1377-1387
-
Konferensbidrag (refereegranskat)abstract
- Articulated haulers are suited for any kind of transports in difficult terrain. The concept was developed from an agricultural tractor in the 1960’s by Volvo. The main features are the hydraulic steering, the individual front and rear frames connected with a rotational hitch joint, all-wheel drive and large tires. These features give the extraordinary terrain capability. Complete vehicle simulations support development of stability, handling and ride comfort properties during early development of the hauler. Complete vehicle simulations can also be utilized to extract design loads. This paper presents the current status in this field.The complete vehicle MBS simulation model includes flexible bodies, hydraulic system, tires and powertrain. A refined powertrain model in Simulink is used for the analysis presented. The environment is defined by the road and the driver. A comfort track road is used for comfort evaluations and a digitalized endurance test road for load predictions. The driver model aims to follow the given target path and velocity.Simulation and measurement signals are compared by means of relevant spectra; load ranges, level crossings and Power Spectral Density. A scalar measure is defined by the equivalent value to carry out comparisons between simulations and test. Fatigue load simulations need an accurate dynamic description of the vehicle, capturing low frequency excitation. The results shows that the load simulation captures the frequency peaks below 15 Hz, although there are some differences in magnitude and peak frequency. When simulated and measured equivalent values are compared most entities are in the range between 1/2 and 2 regarding estimated life. Further improvements are possible but the simulated load quality should also be related to other uncertainties in the fatigue evaluation process.
|
|
| 2. |
- Nilsson, Börje, et al.
(författare)
-
A method for under bark detection of the wood grain angle radial dependence
- 2007
-
Ingår i: Wood Material Science and Engineering. - : Taylor & Francis. - 1748-0272. ; 2:3,4, s. 118-129
-
Tidskriftsartikel (refereegranskat)abstract
- Twist in wood, being closely related to spiral grain, may cause serious problems in building structures, furniture, and joinery. It is therefore of great interest to sort out, at an early stage in the manufacturing process, trees, logs and boards that have an access of spiral grain. The spiral grain pattern is described by a helical deviation of the fibre direction in relation to the longitudinal direction of a living tree or a log and seems to be an indicator for other defects such as compression wood. Remote microwave sensing of spiral grain has received a large interest during the latest two decades. Its development has been impeded by the large variation with moisture content of the microwave properties of wood and by the complexity in modelling the electromagnetic field in a log with spiral grain. A review is presented of a direct method with no requirement of information on moisture content for boards. This procedure has recently been generalized to cylindrical logs and trees having a constant slope of the grain. A further generalization is presented here to allow for the normal spiral grain pattern with radially changing slope of grain in wood under bark. Based on this theory, a measurement procedure is proposed for the detection of wood grain angle with radial dependence, requiring no information on moisture content in the sapwood, also applicable for completely or partially frozen wood. A suitable application would be an instrument to use in the forest for measurements on living trees or logs.
|
|
| 3. |
- Nilsson, Börje, et al.
(författare)
-
Estimation of Twist in Uniaxial Cylinders WithInverse Electromagnetic Scattering
- 2009
-
Ingår i: IEEE Transactions on Antennas and Propagation. - 0018-926X .- 1558-2221. ; 57:10, s. 3264-3273
-
Tidskriftsartikel (refereegranskat)abstract
- For the purpose of determining the twist of a homogeneous, locally reacting, uniaxial cylinder, an inverse microwave scattering theory is presented. Remote measurements of the spiral grain of trees and logs are the prime application. Based on practical considerations, it is assumed that the transmitting and receiving antennas are collocated, requiring a three-dimensional modelling. A general theory is first developed, followed by an asymptotic analysis assuming that the distance from the antennas to the cylinder is many wavelengths and many cylinder radii. In this way, a substantial reduction of the numerical complexity, to the level of the two-dimensional case, is achieved. The error of the determined twist angle as function of inherent parameters of the problem using a CramEacuter-Rao analysis is given. The results from numerical simulations show that this error is low enough for determining the grain angle. Presented parameter studies of the error can be used for minimizing the errors in a measurement set up, of particular interest for non-sophisticated instruments and non-ideal laboratory conditions, by selecting optimum parameters such as frequency and antenna gain. Finally, it is stated that the model has a great potential for developing efficient algorithms for measuring the twist angle.
|
|
| 4. |
- Nilsson, Börje, et al.
(författare)
-
Inverse scattering for determination of twist in uniaxial cylinders
- 2007
-
Ingår i: EMB 07 — Proceedings. - : Lunds universitet. ; , s. 177-184
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- An inverse microwave scattering theory is presented for the determination of twist of a homogeneous locally reacting uniaxial cylinder. The prime application is remote measurements of spiral grain of trees and logs. From practical considerations, the transmitting and receiving antennas are near each other requiring a three-dimensional modelling. First a general theory is developed. Then asymptotic formulae are derived assuming that the distance from the antennas to the cylinder is many wavelengths and many cylinder radii granting a substantial reduction of the numerical complexity compared to the twodimensional case. Finally, the error of the determined twist angle as function of inherent parameters of the problem is given using a Cramer Rao analysis.
|
|
| 5. |
- Nordebo, Sven, et al.
(författare)
-
An adjoint field approach to Fisher information-based sensitivity analysis in electrical impedance tomography
- 2010
-
Ingår i: Inverse Problems. - : IOP Publishing. - 1361-6420 .- 0266-5611. ; 26:12
-
Tidskriftsartikel (refereegranskat)abstract
- An adjoint field approach is used to formulate a general numerical framework for Fisher information-based sensitivity analysis in electrical impedance tomography. General expressions are given for the gradients used in standard least-squares optimization, i.e. the Jacobian related to the forward problem, and it is shown that these gradient expressions are compatible with commonly used electrode models such as the shunt model and the complete electrode model. By using the adjoint field formulations together with a variational analysis, it is also shown how the computation of the Fisher information can be integrated with the gradient calculations used for optimization. It is furthermore described how the Fisher information analysis and the related sensitivity map can be used in a preconditioning strategy to obtain a well-balanced parameter sensitivity and improved performance for gradient-based quasi-Newton optimization algorithms in electrical impedance tomography. Numerical simulations as well as reconstructions based on experimental data are used to illustrate the sensitivity analysis and the performance of the improved inversion algorithm in a four-electrode measurement set-up.
|
|
| 6. |
- Nordebo, Sven, et al.
(författare)
-
Data fusion for electromagnetic and electrical resistive tomography based on maximum likelihood
- 2011
-
Ingår i: International Journal of Geophysics. - : Hindawi Limited. - 1687-885X .- 1687-8868. ; 2011
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a maximum likelihood based approach to data fusion for electromagnetic (EM) and electrical resistive (ER) tomography. The statistical maximum likelihood criterion is closely linked to the additive Fisher information measure, and it facilitates an appropriate weighting of the measurement data which can be useful with multiphysics inverse problems. The Fisher information is particularly useful for inverse problems which can be linearized similar to the Born approximation. In this paper, a proper scalar product is defined for the measurements and a truncated Singular Value Decomposition (SVD) based algorithm is devised which combines the measurement data of the two imaging modalities in a way that is optimal in the sense of maximum likelihood. As a multiphysics problem formulation with applications in geophysics, the problem of tunnel detection based on EM and ER tomography is studied in this paper. To illustrate the connection between the Green's functions, the gradients and the Fisher information, two simple and generic forward models are described in detail regarding two-dimensional EM and ER tomography, respectively.
|
|
| 7. |
|
|
| 8. |
- Nordebo, Sven, et al.
(författare)
-
Fisher information analysis and preconditioning in electrical impedance tomography
- 2010
-
Ingår i: Journal of Physics: Conference Series 224 (2010), 1742-6588. - : IOP Publishing. - 1742-6596 .- 1742-6588. ; , s. 012057-
-
Konferensbidrag (refereegranskat)abstract
- An adjoint field approach is used to formulate a general numerical framework for Fisher information based sensitivity analysis in electrical impedance tomography. General expressions are given for the gradients used in standard least squares optimization, i.e., the Jacobian related to the forward problem, and it is shown that these gradient expressions are consistent with commonly used electrode models such as the shunt model and the complete electrode model. By using the adjoint field formulations together with a variational analysis, it is also shown how the computation of the Fisher information can be integrated with the gradient calculations used for optimization. It is furthermore described how the Fisher information analysis and the related sensitivity map can be used in a preconditioning strategy to obtain a well balanced parameter sensitivity and improved performance for gradient based quasi-Newton optimization algorithms in electrical impedance tomography. Numerical simulations as well as reconstructions based on experimental data are used to illustrate the sensitivity analysis and the performance of the improved inversion algorithm in a four-electrode measurement set-up.
|
|
| 9. |
- Nordebo, Sven, et al.
(författare)
-
Fisher information analysis in electrical impedance tomography
- 2012
-
Ingår i: Geophysical Research Abstract. ; , s. 2012-3553-1-
-
Konferensbidrag (refereegranskat)abstract
- In this contribution it is demonstrated how the Cramér-Rao lower bound provides a quantitative analysis of theoptimal accuracy and resolution in inverse imaging, see also Nordebo et al., 2010, 2010b, 2010c. The imagingproblem is characterized by the forward operator and its Jacobian. The Fisher information operator is defined fora deterministic parameter in a real Hilbert space and a stochastic measurement in a finite-dimensional complexHilbert space with Gaussian measure. The connection between the Fisher information and the Singular ValueDecomposition (SVD) based on the Maximum Likelihood (ML) criterion (the ML-based SVD) is established. Itis shown that the eigenspaces of the Fisher information provide a suitable basis to quantify the trade-off betweenthe accuracy and the resolution of the (non-linear) inverse problem. It is also shown that the truncated ML-basedpseudo-inverse is a suitable regularization strategy for a linearized problem, which exploits a sufficient statisticsfor estimation within these subspaces.The statistical-based Cramér-Rao lower bound provides a complement to the deterministic upper bounds and theL-curve techniques that are employed with linearized inversion (Kirsch, 1996; Hansen, 1992, 1998, 2010). To thisend, the Electrical Impedance Tomography (EIT) provides an interesting example where the eigenvalues of theSVD usually do not exhibit a very sharp cut-off, and a trade-off between the accuracy and the resolution may be ofpractical importance. A numerical study of EIT is described, including a statistical analysis of the model errors dueto the linearization. The Fisher information and sensitivity analysis is also used to compare, evaluate, and optimizemeasurement configurations in EIT.
|
|
| 10. |
- Nordebo, Sven, et al.
(författare)
-
Fisher information analysis in electrical impedance tomography
- 2013
-
Ingår i: Journal of Geophysics and Engineering. - : Institute of Physics (IOP). - 1742-2132 .- 1742-2140. ; 10:6, s. Article ID: 064008-
-
Tidskriftsartikel (refereegranskat)abstract
- This paper provides a quantitative analysis of the optimal accuracy and resolution in electrical impedance tomography (EIT) based on the Cramér–Rao lower bound. The imaging problem is characterized by the forward operator and its Jacobian. The Fisher information operator is defined for a deterministic parameter in a real Hilbert space and a stochastic measurement in a finite-dimensional complex Hilbert space with a Gaussian measure. The connection between the Fisher information and the singular value decomposition (SVD) based on the maximum likelihood (ML) criterion (the ML-based SVD) is established. It is shown that the eigenspaces of the Fisher information provide a suitable basis to quantify the trade-off between the accuracy and the resolution of the (nonlinear) inverse problem. It is also shown that the truncated ML-based pseudo-inverse is a suitable regularization strategy for a linearized problem, which exploits sufficient statistics for estimation within these subspaces. The statistical-based Cramér–Rao lower bound provides a complement to the deterministic upper bounds and the L-curve techniques that are employed with linearized inversion. To this end, electrical impedance tomography provides an interesting example where the eigenvalues of the SVD usually do not exhibit a very sharp cut-off, and a trade-off between the accuracy and the resolution may be of practical importance. A numerical study of a hypothetical EIT problem is described, including a statistical analysis of the model errors due to the linearization.
|
|
